Mesh not showing in game engine after lattice deform
Lattice is not supported By the BGE. 

Unsupported objects (curve, particle ,lattice ....) will not be converted to game objects when starting the game session. This means they will not present within the game.

Be aware children of unsupported objects will not appear as the parent is not present in the scene. I think this is your current situation.

I suggest to remove the lattice completely. At least unparent the cube from it.